Embryogenesis of the rat heart: the expression of collagenases

Little is known about extracellular matrix (ECM) remodeling during heart development. Matrix degrading metalloproteinases are possible candidates contributing to degradation of ECM during these complex biological events. We described here different forms of MMPs, based on their substrate specificity, molecular weight, immunolocalization and in situ zymography within embryonic rat myocardium at different stages of heart development (from embryonic day - ED12 until ED21). Murine collagenase-3 (MMP-13), stromelysin (MMP-3) and gelatinases A&B (MMP-2 & -9) were expressed in prenatal hearts, as demonstrated by quantitative zymography and immunohistochemistry. MMP-2, -3 and -9 were found within myocardium of avascular (ED12) and vascularized heart (ED14-21). An extensive immunolabeling over the heart trabeculae, epicardial tissue and a weaker labeling in the endocardial and truncoconal cushion tissue was observed at all stages of the heart development. Utilizing quantitative zymography we found that MMP-13 activity gradually increased from ED14-ED16 reaching a plateau from ED16-ED21, while MMP-2 activity demonstrated a transient increase starting at ED13, peaked at ED16 and declined thereafter. As to MMP-9 activity, it was seen only between ED16 and ED 18. In situ zymography with gelatin as a substrate represented activity of MMPs within the myocardium of the atria and the ventricles and a very strong activity in the interstitial tissue of the endocardial and the conotruncal cushion tissue.

MMPs expressed in embryonic heart correspond to all major classes of these enzymes. They may contribute to embryonic remodeling of the heart.